An electric double layer capacitor includes a capacitor element having a polarizable electrode as a positive electrode, another polarizable electrode as a negative electrode, and a separator provided between these electrodes. The polarizable electrodes face each other across the separator. The capacitor element is impregnated with a driving electrolyte.
A polarizable electrode generally contains activated carbon. The surface of the activated carbon has a functional group, such as a carboxyl group, a phenolic hydroxyl group, a carbonyl group, or a quinone group, provided thereon. These surface functional groups influence the characteristics of the capacitor. An electric double layer capacitor including polarized electrode layers containing activated carbon with abundant acidic surface functional groups (carboxyl group, carbonyl group, phenolic hydroxyl group, lactone group) has a large site contributing to the capacitance of the capacitor, accordingly having a large pseudocapacitance.
A conventional electric double layer capacitor disclosed in Japanese Patent Laid-Open Publication No. 2000-169129 has the total amount of at least one surface functional group selected from a carboxyl group, a quinone group, a hydroxyl group, and a lactone group ranging from 0.2 mEq to 1.00 mEq per 1 gram of activated carbon, thereby having a large capacitance. This capacitor includes, as a driving electrolyte, a solution containing an aprotic polar solvent, such as propylene carbonate, and a quaternary ammonium salt dissolving in the solvent.
An electric double layer capacitor disclosed in Japanese Patent Laid-Open Publication Nos. 11-54376 and 11-54379 includes, as a driving electrolyte, a solution containing an aprotic polar solvent, such as propylene carbonate, and an amidine salt dissolving in the solvent. This solution provides a capacitance larger than that of a capacitor employing quaternary ammonium salt. However, these documents do not address characteristics at low temperatures, while they address characteristics at a room temperature.
A conventional electric double layer capacitor including a driving electrolyte containing an aprotic polar solvent, such as propylene carbonate, and an amidine salt dissolving in the solvent, and a polarizable electrode layer containing an activated carbon having a sufficient amount of acidic surface functional groups has a large pseudocapacitance. However, the driving electrolyte electrochemically reacts with the acidic surface functional groups on the surface of the polarizable electrode layer during a charge and discharge reliability test, thereby producing a large amount of reactant. This reactant blocks the site of the activated carbon contributing to the capacitance, hence reducing the capacitance and deteriorating the characteristics, e.g. increasing its resistance. This deterioration prominently appears particularly at low temperatures.